1. Field of the Invention
This invention relates to the construction of a package of a semiconductor device, and more particularly to a package on which a large number of external terminals are arranged.
2. Description of the Related Art
A Pin Grid Array (PGA) package is known as a package having a large number of external terminals arranged therein. The PGA package is a package having pin-shaped external terminals arranged at regular intervals in a grid form on the rear surface of the package body and formed to vertically project from the rear surface thereof. The semiconductor device using the PGA package is mounted on a printed circuit board having through holes formed therein. In an actual case, the external terminals (external pins) of the PGA package are inserted into the through holes of the printed circuit board and soldered.
In recent years, in the PGA package, the number of external pins has been increased in order to attain more complicated multifunctions and a larger added value of the semiconductor device. When the number of external pins becomes as high as several hundreds, the size of the PGA package becomes significantly large. When the size of the PGA package becomes large with an increase in the number of pins, there may occur problems in that the cost of the package body is raised, the weight thereof is increased, and the electrical characteristics are degraded (for example, the signal transmission distortion and the power source noise are increased) by an increase in inductance, capacitance and resistance components with an increase in the length of wirings in the package.
In a case where the size of the package body is reduced in order to suppress the influence of the above problems, it becomes important to reduce the pitch of the external pins. Since it becomes necessary to reduce the pin diameter with a decrease in the pitch, the external pins are liable to be bent at the time of the mounting of the package body on the printed circuit board, the mounting precision of the external pins cannot be enhanced, and the mounting operation becomes difficult. Further, when the pin diameter is reduced, it becomes, necessary for the manufactures of the packages to effect visual inspection of all of the products so as to check whether or not the external pins are bent or the products satisfy the specification, giving a bad influence on mass-productivity and increasing the manufacturing cost. When the pin diameter is reduced, a bridge of the solder may be made at the time of mounting of the external pins on the printed circuit board.
Thus, the conventional PGA package has various problems when the number of external pins is increased. Further, in a case where the PGA package is inserted into the through hole formed in the printed circuit board and mounted on the printed circuit board, it becomes difficult to reduce the size of an electronic instrument using the printed circuit board and it becomes difficult to apply the package to a portable type electronic instrument.